U.S. Pat. No. 4,087,394 (Concannon) discloses an aqueous dispersion of certain fluoropolymers, which dispersion contains dissolved film-forming material (binder) for forming a non-stick coating. Example 1 discloses the preparation of such a dispersion, containing 51 wt % TFE/HFP copolymer, 32 wt % polyamide acid salt (also called polyamic acid), 13.9 wt % carbon black pigment, and 2.3 wt % aluminum silicate extender supplied as part of the black pigment mill base and present as a grinding aid therein. The weight proportion of fluoropolymer to binder was 1.6:1. This dispersion was applied to an aluminum sheet and baked to a coating having excellent release properties. In the baking process, the coating composition is disclosed to tend to stratify, with the binder concentrating at the substrate surface to provide adhesion of the coating to it, and the fluoropolymer concentrating at the outer surface to provide the release property.
The aqueous dispersion approach of Concannon for creating the non-stick coating has been widely adopted for the creation of primer layers for overcoating with at least one fluoropolymer layer to produce non-stick finishes. In the primer application, the fluoropolymer concentrating at the top surface of the primer layer provides inter-coat adhesion to the fluoropolymer overcoat.
With the use of the Concannon approach in providing primer coating, several additional measures were taken. First, for improved release and easy clean, higher film builds (thickness) for the overcoat, i.e. a midcoat and a topcoat, each being primarily fluoropolymer, were used. Second, to obtain good intercoat adhesion between the primer layer and the midcoat, the weight ratio between fluoropolymer and binder in the primer layer was kept high, 2.4 and higher. Third, to obtain adequate adhesion of the primer layer, the substrate was roughened, e.g. by grit blasting when the substrate was metal and pre-coating of the substrate with a glass flit when the substrate was ceramic, e.g. enamel. This toughening provided a mechanical bond of the primer to the substrate in addition to the adhesion provided by the binder component of the primer layer.
In addition to providing improved adhesion, it became desirable to improve durability of the non-stick coating, and this was accomplished by adding inorganic filler film hardener, such as clay, to the fluoropolymer overcoat.
It became desirable to eliminate the step of roughening the substrate, which would require the primer to have the sole responsibility for achieving adhesion. Recent patents disclose the achievement of providing primers for non-stick finishes on certain smooth substrate surfaces, by controlling the stratification occurring during baking of the primer layer. U.S. Pat. Nos. 5,168,013 and 5,240,775 (both by Tannenbaum) disclose that the fluoropolymer component should be composed of polytetrafluoroethylene (PTFE) of low and high melt viscosity or of PTFE and a TFE/PAVE copolymer, respectively. PCT publication WO 94/14904 (Thomas) discloses the fluoropolymer component to be two different perfluorocarbon resins generally having lower melt viscosity than the Tannenbaum PTFE and the binder component to be two different binders in certain ratios. The primers prepared in these references were applied to smooth degreased aluminum and had fluoropolymer to binder weight ratios of 2.5, 2.4, and 1.5, respectively.
This adhesion to certain smooth surfaces was achieved by not having filler film hardener present in the primer composition in any amount which could noticeably increase primer layer durability, with the attendent adverse effect of adhesion to the substrate. In Example 1 of Thomas the aluminum silicate is present as an extender (grinding aid) for the carbon black pigment, and its amount is only 3.2 wt % based on solid material present. The colloidal silica component (7.4 wt %) an adhesion promoter, has also been used as a binder (U.S. Pat. No. 4,118,537 to Vary et. al.), and indeed this component is not used as a filler film hardener; it has a much finer particle size than filler, viz. less than 0.1 micrometer as compared to filler particles which are generally at least one micrometer in size (colloidal silica in water looks like a clear liquid).
When filler film hardener was added to the primer layer composition to toughen the overall non-stick finish by the primer layer resisting scratches which penetrated the overcoat, the primer layer would no longer adhere adequately to a smooth substrate. In a commercial primer having a fluoropolymer/binder weight ratio of 2.4:1, incorporation of aluminum oxide film hardener into the primer reduced the adhesion to the substrate such that the substrate had to be roughened to achieve adequate adhesion (U.S. Pat. No. 5,250,356 to Batzer).
The need remains for a primer for non-stick coatings which has both durability, adequate adhesion to smooth substrate, and adequate adhesion to a fluoropolymer overcoat.